Most disposable cartridge filters are usually operated with outside-to-inside flow direction of the liquids or gases to be filtered. The outside-to-inside flow permits a greater dirt-holding volume in the filter housing than if inside-to-outside flow were used. In addition, it is easier to support the filter tube to resist high differential pressures when the flow direction is outside-to-inside, because the smaller-diameter mechanical support (usually a perforated tube or porous rigid tube) can usually be fabricated from commercially available materials, while a larger-diameter support for the outside of the cartridge usually requires custom fabrication.
There is another reason that internal support is usually preferred over external support, which applies particularly to disposable cylindrical filter tubes; for example, with approximately 1/8-inch wall thickness, made from a nonwoven random network of glass fibers 0.1 to 10 microns in diameter and bonded at the junction of the fibers by a hardened material, such as a resin. Filter tubes of this general type are described, for example, in U.S. Pat. No. 3,767,054, issued Oct. 23, 1973, hereby incorporated by reference. Filter tubes of this type are commonly manufactured by forming onto the external surface of a cylinder, either by depositing fibers onto a porous cylinder wall by vacuum, or by rolling a sheet of fibers onto the wall of the cylinder. As a result, the inside diameter of the filter tube is almost exactly the same as the outside diameter of the forming cylinder, and, therefore, the inside diameter of the filter tubes can be made uniform and generally reproducible from tube to tube in production. For example, a control range of .+-. 0.005 inches on the tube internal diameter, or even less, is easily achieved. However, since the outside diameter of the filter tube is not confined during the forming process, control of the diameter is much more difficult, and the variability in outside diameter from tube to tube is much greater, typically .+-. 0.030 inches or more.
The variability in external diameter of filter tubes of this type has made the design and fabrication of external supports much more difficult and expensive than internal supports. For example, a reusable internal support of perforated metal or plastic (as shown in U.S. Pat. No. 3,767,054) is quite feasible, because the close control of internal diameter of the filter tubes assures that each filter tube will fit the support perfectly. It will be appreciated that satisfactory performance of the support requires that the internal diameter of the filter tube be large enough to permit the tube to be fitted; e.g., slid, over the support easily, and yet the internal diameter of the filter tube must not be considerably larger than the support core or else the support core will not adequately prevent rupture of the filter tube.
With the wide variability of outside diameters of the filter tubes, it has been impractical to design a reusable external support core. A support core which is large enough to fit over filter tubes at the large end of the diameter range will be too loose to support filter tubes at the small end of the diameter range. Even a disposable external support core is difficult and expensive, because the support core must be strong enough to prevent the filter tube from bursting, yet malleable enough to conform closely to the variable outside diameter dimensions of the filter tube. The external support problem may be solved by forming or casting the filter tube inside a rigid perforated or porous support, such as a screen, but this manufacturing procedure is inherently more expensive and difficult than forming the filter tube on the outside of a cylinder or mandrel.
Despite the difficulties in providing adequate dirt-holding capacity or burst strength for the filter tubes, there are definite advantages to filtering fluids in the inside-to-outside direction under certain conditions and for certain purposes. For example, when coalescing and removing liquid droplets from air or other gases, the inside-out flow direction is essential to permit drainage of the coalesced liquid and minimize the chance of reentrainment and carryover of coalesced liquid by the gas (see Hydraulics and Pneumatics, August 1974, "Coalescing Filters Produce Clean Air for Fluidic Systems" (herein incorporated by reference).
Another example in which inside-out flow is desirable is in coalescing and separating of two or more liquid phases. In this procedure, the filter collects and coalesces droplets of the dispersed liquid phase and produces two distinct liquid phases which may then be easily separated. However, the coalescing action takes place throughout the depth of the filter tube element, and the clean separation of the two liquid phases occurs on the downstream surface of the filter tube. If the flow direction is outside-to-inside, the acceleration of the liquid as it leaves the relatively small flow area inside the tube tends to mix the two liquid phases and redisperse the discontinuous phase. However, if flow is inside-to-outside, the liquid leaves the filter surface at minimum velocity, and the two liquid phases can separate cleanly within the filter housing.
A further example in which inside-to-outside flow is desirable is when a two-stage treatment of the fluid is desirable; for example, contacting the liquid with a sorbent material, such as an adsorbent clay or diatomaceous earth, followed by filtration. If flow is inside-to-outside through the filter tube, the loose powder, granular, or fibrous material which is used for the sorbent pretreatment, either as a filter aid or precoat material, or both, may be conveniently preloaded into the inside of the filter tube, and if necessary, held in place with perforated end caps. The single disposable filter tube cartridge then conveniently serves both to pretreat and filter the fluid. If flow were outside-to-inside through the filter tube, it would be difficult or impossible to retain a precoat of powder, granular, or fibrous material on the outside of the filter tube, and therefore a less convenient two- or multiple-step filter process would be required, such as is described in Bulletin TI-62, 1973, of Balston, Inc. (hereby incorporated by reference).